Background: Although canine clinical manifestations of Lyme disease vary widely, cutaneous manifestations are not well documented in dogs. In contrast, a variety of cutaneous manifestations are reported in human Lyme disease caused by the spirochete Borrelia burgdorferi. A recently recognized dermopathy associated with tickborne illness known as Morgellons disease is characterized by brightly-colored filamentous inclusions and projections detected in ulcerative lesions and under unbroken skin. Recent studies have demonstrated that the dermal filaments are collagen and keratin biofibers produced by epithelial cells in response to spirochetal infection. We now describe a similar filamentous dermatitis in canine Lyme disease. Methods and Results: Nine dogs were found to have cutaneous ulcerative lesions containing embedded or projecting dermal filaments. Spirochetes characterized as Borrelia spp. were detected in skin tissue by culture, histology, immunohistochemistry, polymerase chain reaction (PCR) and gene sequencing performed at five independent laboratories. Borrelia DNA was detected either directly from skin specimens or from cultures inoculated with skin specimens taken from the nine canine study subjects. Amplicon sequences from two canine samples matched gene sequences for Borrelia burgdorferi sensu stricto. PCR amplification failed to detect spirochetes in dermatological specimens from four healthy asymptomatic dogs. Conclusions:Our study provides evidence that a filamentous dermatitis analogous to Morgellons disease may be a manifestation of Lyme disease in domestic dogs.